Bright annealing of tubular metal articles



March 19, 1968 A..C. L|NGENFELTER 3,374,126

BRIGHT ANNEALING OF TUBULAR METAL ARTICLES Filed Oct. 8, 1965 INVENTOR. flue/v C Z/fiGEI/FEAJEQ 477OPNEY United States Patent ()fifice 3,374,126 Patented Mar. 19, 1968 3,374,126 BRIGHT ANNEALING F TUBULAR METAL ARTICLES Allen C. Lingenfelter, Ona, W. Va., assignor to The International Nickel Company, Inc., New York, N.Y.,

a corporation of Delaware Filed Oct. 8, 1965, Ser. No. 494,211 4 Claims. (Cl. 148-203) ABSTRACT OF THE DISCLOSURE In providing a bright annealed surface on the interior of a metal tube a plug is blown through the tube with a non-oxidizing gas, thereafter a non-oxidizing atmosphere is maintained in the tube, the ends of the tube are annealed, forced closed, surface ground and sealed by resistance welding and the tube is then thermally annealed with the non-oxidizing atmosphere sealed therein.

The present invention relates to heat treatment of metals, including alloys, and, more particularly, to obtaining bright annealed metallic surfaces.

It is well known that many metals, e.g., carbon steel, stainless steel and some nickel alloys, undergo surface oxidation during annealing heat treatments when the heat treatment is performed with the metal in an atmosphere of air or other oxidizing gas. Such oxidation usually results in scale or discoloration at the surface of the metal and in some instances is detrimental to the corrosion resistance of the metal surface. Results obtained by annealing in an oxidizing atmosphere and thereafter mechanically or chemically cleaning, e.g., grit blasting, grinding or pickling, the metal surface are not always entirely satisfactory for obtaining highly corrosion resistant metal surfaces, especially metal surfaces for use in association with fluids which must be maintained in a high purity condition. Oxidation during annealing may penetrate too deeply to be completely removed by commercially practical amounts of grinding, blasting and pickling. Also, grit blasting sometimes results in particles of oxide or other foreign matter embedding in the metal surface. In some instances pickling methods are too expensive or have detrimental corrosive effect. Grinding is often not commercially practical for removing oxides from some metal surfaces such as interior surfaces in long tubes of small diameter, e.g., 1 inch or less diameter. Furthermore, inasmuch as mechanical cleaning processes can result in strained and possibly work hardened microlayers at the metal surface and thus can be detrimental to corrosion resistance, purchasers of articles for corrosion resistant use sometimes require that the articles be provided in a bright-annealed condition. Bright-annealed surfaces are understood to be metallic surfaces which are characterized in the as-annealed condition by having a metallic luster and by being essentially free of visible oxides.

Heretofore, bright-annealed surfaces have been obtained by annealing in non-oxidizing atmospheres. For bright annealing metal articles (including metal products such as sheet, bar and tubing) in a non-oxidizing atmo phere, it has been generally necessary to provide a furnace with a special chamber which is capable of being sealed with the non-oxidizing atmosphere and the metal articles inside the chamber during annealing or else to provide apparatus for maintaining a flow of non-oxidizing gas over thearticles during annealing. However, the apparatus needed for known bright-annealing processes is expensive to provide and maintain. In some instances, especially where the articles to be annealed are of large dimension, the financial investment and the maintenance and operating costs required for employment of known bright-annealing processes is so great that bright annealing by known processes is not commercially feasible. A particular problem arises where long lengths of tubing for carrying corrosive fluids, e.g., 20-foot or 60-foot or longer tubes of stainless steel or nickel-base alloys, must be annealed without oxidizing the interior surfaces thereof and yet at least a moderate degree of oxidation is permissible on the exterior. Thus, in such instances, providing special bright-annealing furnaces adapted for maintaining protective atmospheres around long tubesrequires a high investment in apparatus for providing more protection than is actually needed. Moreover, when long tubes are to be bright annealed, particularly when annealing many tubes at once in a batch-type operation, problems arise due to difficulties of removing air from the interiors of the tubes. Until now there has been an unfilled need for an improved, commercially practical, economic process whereby a bright-annealed surface can be obtained at the interior of long tubes by annealing the tubes in an air atmosphere or a commercial gas-fired furnace atmosphere without need for a protective medium around the tubes.

Although many attempts were made to overcome the foregoing difiiculties and other disadvantages, none, as far as I am aware, was entirely successful when carried into practice on an industrial scale.

It has now been discovered that a bright-annealed surface can be provided on the interior of a tubular metal article when the article is annealed in a commercial gas fired furnace atmosphere or an air atmosphere without a protective medium around the tube.

It is an object of the present invention to provide a new process for annealing metal tubes and other hollow metal articles of substantially uniform interior cross-section and preventing oxidation of the interior surfaces thereof during annealing.

It is a further object of the invention to provide a new process for annealing nickel-chromium iron alloy tubing and obtaining a bright-annealed finish on the interior of the tubing.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a cross-sectional view of a tube being filled with non-oxidizing gas in a process within the scope of the invention;

FIGURE 2 illustrates, in an exterior view, a tube end which has been closed in accordance with the invention; and

FIGURE 3 depicts, in an exterior view taken at to the view in FIGURE 2, a tube end closure which has been scaled by resistance welding in accordance with the invention.

Generally speaking, the present invention is directed to a process for annealing a metal tube or other tube-like article and obtaining a bright-annealed surface on the interior of the tube, which process comprises displacing the air from a metal. tube and concurrently filling the tube with a non-oxidizing gas at a pressure greater than the air pressure in the tube while at the same time maintaining a moving barrier between the gas and the air to thereby prevent mixing of the gas and the air in the tube, thereafter sealing the ends of the tube, i.e., the end portions which are near or at the extremitiesof the tube, thermally annealing the tube, cooling the tube and then removing the sealed ends of the tube. When displacing the air from the tube, a close fitting plug which serves as a moving diffusion barrier is forced through the tube by pressure of a non-oxidizing gas behind the plug. Thus, the plug displaces air from the tube and concurrently the tube is filled with a non-oxidizing atmosphere. Thereafter, a closure is made at each end of the tube by forcing the tube wall against itself at (or near) each end of the tube and the end closures are sealed gas-tight, advantageously by resistance Welding, meanwhile maintaining a nonoxidizing atmosphere Within the tube. It is essential that the closures be completely sealed with suf ficient gas-tightness and strength to prevent entrance of any air or other oxidizing gas into the tube when the tube is heated and cooled between room temperature and the annealing temperature of the tube and resistance welding is especially advantageous for efficiently and safely sealing the closures with the required gas-tightness and strength. The sealed tube having a I'lOl'l-OXlCllZlng atmosphere within is thermally annealed by an annealing treatment which can be performed by heating the tube in the atmosphere of a furnace having an oxidizing atmosphere such as an air atmosphere or a gas-fired furnace atmosphere. After annealing, the tube is cooled to a temperature below the lowest temperature at which the tube material readily oxidizes (minimum oxidation temperature) e.g., forms visible oxide films in air, and the sealed ends of the tube are then removed. Thus, the process of the invention provides a bright-annealed finish on the interior of a metal tube without need for enclosing the entire tube in an inert atmosphere.

The close fitting plug which in accordance with the invention is forced through the tube when displacing the air therein, is of a size and material adapted to fit snugly in contact with the interior wall of the tube and prevent substantial amounts of gas or air from passing around or through the plug without having high frictional resistance to movement of the plug through the tube. Felt is an especially advantageous material for the plug although other materials, e.g., packed cotton, having similar characteristics of moderate compressibility, expansility and resiliency and of low gas permeability, e.g., substantially impermeable, can be employed. The resiliency and other aforementioned characteristics of the plug enable the plug to conform to the interior surface of a tube which is substantially but not perfectly uniform in internal crosssection throughout the length of the tube and thus enable the plug to act effectively as a moveable seal when traversing the tube in accordance with the invention. For use with cylindrical tubing, the plug, when not compressed, characteristically has a circular cross section with a diameter slightly greater than the internal diameter of the tube. Accordingly, the plug can be of cylindrical, conical or sperical configuration. Advantageously, all dimensions of the plug are slightly greater than the internal diameter of the tube so that whatever the orientation of the plug in the tube, the plug will effectively seal the cross section of the tube. Displacing the air from a tube by forcing a plug through the tube in accordance with the invention is especially effective for quickly and efficiently providing the required atmosphere in the tube and avoiding excess loss of non-oxidizing gas by prolonged purging.

Non-oxidizing gases which are satisfactory for the process of the invention include hydrogen, dissociated ammonia and argon. The atmosphere which is sealed within the tube must be dried to a low dew point and be of a commercial purity consistent with atmospheres used in good bright-annealing practice. Accordingly, the atmosphere must be extremely low in, or devoid of, contaminants having an appreciable oxidizing or corrosive effect on the tube. Good results are obtained using dry hydrogen having a dew point not higher than minus 60 F. as the non-oxidizing gas when annealing nickel-chromiumiron alloy tubes by the process of the invention.

In carrying the invention into practice to anneal and thereby soften a metal tube which has been hardened by cold working or otherwise, it is advantageous, before closing the ends of the tube, to at least partially anneal the hardened tube at the places where end portions of the tube wall are to be closed so that the metal at these places is in a ductile condition which can be readily formed into a good closure with opposing interior surfaces of the tube wall in close proximity to each other. In order to avoid oxidation of the interior surface of the tube, a non-oxidizing atmosphere is established within the tube before annealing the closure portions and a flow of non-oxidizing gas is maintained through the tube until the exit end of the tube, i.e., the end through which the gas exits, is sealed. The tube is closed, first near the exit end, by forcing the tube wall against itself. Oxide films or other foreign matter, if present, should be removed from the exterior of the closure, e.g., by grinding, in order to have good electrical conductivity for resistance welding. The exit end closure is sealed by making a continuous resistance weld across the closure. Although gas can no longer flow through the tube when the exit closure is sealed, the gas source is kept connected to the tube and a positive pressure (above surrounding atmospheric pressure) of at least about pound per square inch gage (-p.s.i.g.) of non-oxidizing gas is maintained in the tube after the exit end is sealed in order to avoid entry of air into the tube. The tube is closed and sealed near the entrance end by the same techniques used for closing the exit end, thereby sealing within the tube a non-oxidizing atmosphere at a pressure of at least about /4 p.s.i.g., advantageously about /2 p.s.ig. to about 1 p.s.i.g., at room temperature. Thereafter, the tube is disconnected from the source of nonoxidizing gas and annealed by heating.

Annealing cycles with temperatures and heating periods known to those skilled in the art to be sufficient for satisfactorily softening the tube metal, depending upon the chemical composition and metallurgical condition of the tube, can be employed in the process of the invention. For instance, cold worked tubes of a nickel-chromiumiron alloy containing 14% to 17% chromium, 6% to 10% iron, up to 0.15% carbon and balance substantially nickel are satisfactorily annealed by heating at 1700 F. to 1900 F. for 30 minutes to 15 minutes. After annealing, the tube is cooled, usually to room temperature, and the deformed end portions are then removed by abrasive wheel cutting or by any other means that does not detrimentally oxidize or deform the tube. Annealed tubes which have been processed in accordance with the invention have interior surfaces characterized by a bright-annealed finish which is equal in quality of freedom from oxidation to the quality of bright-annealed finishes obtained by heat treating tubes in non-oxidizing atmospheres of commercial bright annealing furnaces that are far more expensive than furnaces which can be employed for annealing in accordance with the invention.

Among other advantages of the invention, it is to be noted that tubes having non-oxidizing atmospheres sealed therein in accordance with the invention can be prepared in advance of annealing and stored for long periods, e.g., days or weeks, without detriment if required, for instance, to conform with production schedules for annealing heat treatment. Furthermore, the invention provides a commercially efficient process enabling bright annealing large numbers of tubes, e.g., 20 or 30 or more tubes, in a single batch heat treatment or in a single pass through a continuous annealing furnace.

Referring now to the accompanying drawing, FIG. 1 shows, in cross section, cylindrical felt plug 10 disposed within the interior of cylindrical tube 11. Side surface 12 of the plug is in close fitting, low dry-friction contact with interior surface 13 of the tube. When displacing air from the tube and replacing the air therein with non-oxidizing gas, gas which is behind the plug, e.g., to the left of the plug in FIG. 1, and at a pressure higher than the pressure of the air in front of the plug forces the plug to move through the tube (to the right in FIG. 1 as therein shown by the arrow) until the plug is ejected from the exit end of the tube. When the plug is thus driven through the tube, the plug acts as a free piston or ram and sweeps the tube clear of air. FIG. 2 shows tube 11 with the tube Wall flattened against itself to form end closure 14 near tube end extremity 15. FIG. 3 shows tube 11, when viewed at 90 to the view of FIG. 2, with continuous resistance weld 16 which continuously joins the tube wall to itself and makes a heat-resistant gas-tight seal across the tube at closure 14. i

For the purpose of giving those skilled in the art a better understanding of the invention and a better appreciation of the advantages of the invention, the following illustrative example is given:

A cold-drawn tube made of a nickel-chromium-iron alloy of a nominal composition containing 15.8% chromium, 7.2% iron, 0.04% carbon, 0.2% manganese, 0.2% silicon and balance substantially nickel was grit blasted to clean the interior of the tube. The tube was of a substantially cylindrical configuration of 'Vs-inch outside diameter, about 0.05 inch wall thickness and 60-foot length. A cylindrical felt plug about -inch in diameter and about As-inch in length was inserted into one end (the entry end) of the tube. A pressurized source of dry hydrogen having a dew point of about minus 60 F. or lower was connected to the tube by a rubber hose with a hose-fitting attached tightly (gas-tight) on the entry end of the tube. The tube was pressurized at the entry end with a gas pressure of about 40 p.s.i.g. from the hydrogen source and the plug was thereby forced to move rapidly through (traverse) the tube in a short time of about one second or less and was ultimately ejected from the exit end of the tube. After the plug was ejected the gas pressure was lowered to about /2 p.s.i.g. to maintain a flow of hydrogen through the tube, thus further purging the tube and preventing entry of air therein. No oxygen was detected in a test of the tube atmosphere about one minute after the plug was ejected. Thereafter, a portion of the tube about 2 inches long and about 2 inches from the extremity of the exit end was induction annealed and a portion of the tube about 2 inches long and about 6 inches from the extremity of the entry end was also induction annealed. An end closure was then made at the annealed portion of the tube nearest the exit end by mechanically forcing the tube wall against itself between forming dies to flatten the tube at this place so that opposing interior surfaces of the tube wall Were in close proximity, e.g., not more than about -inch apart, with each other. Thereafter, the flattened exterior surfaces of the closure were surface ground to remove any oxides and foreign matter present thereon and provide a clean metal surface with good electrical contact for resistance welding. The exit end closure was then sealed by making a single continuous gas-tight resistance weld across the closure portion while exerting pressure externally on the tube walls to maintain the tube Walls in contact with themselves during resistance welding. Although the flow of hydrogen was stopped by the resistance weld seal, a positive pressure of about p.s.i.g. was maintained within the tube by the hydrogen source connected to the entry end. An entry end closure was then made at the annealed portion nearest the entry end by forcing the tube wall against itself between forming dies to flatten the tube and bring opposing interior surfaces of the tube Wall in close proximity with each other. The flattened exterior surfaces of the entry end closure were then cleaned by surface grinding. The entry end closure was sealed gas-tight by making a continuous resistance weld across the entry end closure, thereby sealing a non-oxidizing atmosphere of hydrogen at a pressure of about A p.s.i.g. inside the tube. The hydrogen source was disconnected from the sealed tube, the tube was placed in a gas-fired furnace and the tube was annealed therein by heating for about 20 minutes at about 1800 F. The atmosphere around the tube in the furnace was a natural gas-air fueled furnace atmosphere containing about 6% to 8% combustibles. The annealed tube was removed from the furnace and air cooled to room temperature and the end closure portions were cut off the tube with a liquid-cooled abrasive wheel. The interior of the tube which had been thus annealed in accordance with the invention was in a very satisfactory bright-annealed condition characterized by a metallic luster, freedom from oxide films and visible amounts of oxide and by high corrosion resistance.

The present invention is applicable to bright annealing interior surfaces of tubes and other tube-like articles composed of metals, including alloys, which are ductile and resistanceweldable (when in an annealed condition). The invention is particularly applicable for annealing hardened tubular metal articles, e.g., work hardened tubes made of nickel-base or iron-base alloys containing 12% to 25% chromium, which are characterized by high annealing temperatures, e.g., 1500 F. and higher, and which characteristically form surface oxides when exposed to oxygen-containing atmospheres for short periods, e.g., 5 minutes or 10 minutes, at temperatures as high as the annealing temperature of the tube. Thus, the invention is applicable for annealing tubes made of metal containing a major proportion (at least 50%) of nickel, iron, cobalt and/or copper with or without additional alloying elements, which metals are exemplified as follows: nickel and nickel alloys, including nickel-chromium alloys, nickel-iron-chromium alloys, nickel-cobalt-chromium alloys, .nickel-iron-cobalt-chromium alloys and age-hardenable nickel alloys containing aluminum, titanium and/or columbium; copper-nickel alloys containing 10% to 90% copper and 10% to 90% nickel; and steels, including stainless steels and low alloy steels. For instance, the process can be employed to produce bright-annealed interior surfaces in tubes made of ductile, resistance-weldable metals containing up to 30% chromium, up to 6% columbium, up to 5% molybdenum, up to 4% aluminum, up to 3% titanium up to 2.5% manganese, up to 1.5% silicon, up to 0.3% c-ar-bon with the balance being metal from the group consisting of nickel, iron, cobalt and copper in an amount at least 50% of the alloy, e.g., an alloy containing 14% to 17% chromium, 6% to 10% iron, up to 1% manganese, up to 0.5% silicon,'up to 0.5% copper, up to 0.15% carbon and 72% to nickel. Moreover, the invention is especially applicable in providing bright-annealed interior surfaces in metal tubing for use in containing, circulating, transmitting, etc., fluid which are corrosive and/or which must be maintained in a high purity condition, e.g., tubing for heat exchangers, refinery apparatus, distillation apparatus, boilers, etc.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A process for providing an annealed metal tube having a bright-annealed surface at the interior of the tube comprising:

(a) providing a metal tube having an entrance end opening and an exit end opening;

(b) inserting into the entrance end opening of the tube a resilient, substantially impermeable plug having a cross section compressible and expansile to conform to the interior surfaces of the tube;

(c) connecting a pressurized source of non-oxidizing gas to the entrance end of the tube;

(d) applying a pressure of non-oxidizing gas from said source to the interior of the tube to force said plug through the tube and to eject the plug from the exit end of the tube, thereby displacing air from the tube and establishing a non-oxidizing atmosphere within the tube, and thereafter;

(e) maintaining a non-oxidizing atmosphere within the tube;

(f) annealing a portion of the tube wall near the exit end of the tube and a portion of the tube wall near the entrance end of the tube, forcing the annealed exit-end portion of the tube wall against itself to form a closure near the exit-end of the tube and surface grinding the exterior surface of the exitend closure to remove oxides therefrom;

(g) sealing the exit-end closure gas-tight by resistance welding the tube wall to itself to produce a continuous gas-tight resistance weld completely across the surfaceground exit-end closure;

(h) forcing the annealed entrance-end portion of 'the tube wall against itself to form a closure near the entrance end of the tube and surface grinding the exterior surface of the entrance-end closure to remove oxides therefrom;

(i) sealing the entrance-end closure gas-tight by resistance welding the tube Wall to itself to produce a continuous gas-tight resistance weld completely across the surface-ground entrance-end closure;

(j) disconnecting the tube from the gas source;

(k) thermally annealing the tube;

(l) cooling the tube below the minimum oxidation temperature of the tube metal; and

(in) removing the closures from the ends of the tube by non-oxidizing means.

2. A process as set forth in claim 1 wherein the plug is made of felt.

3. A process as set forth in claim 1 wherein the tube is made of an alloy containing a major proportion of metal from the group consisting of nickel, iron, cobalt and copper and characterized by an annealing temperature of at least about 1500 F. when in a Work hardened condition.

4. A process as set forth in claim 1 wherein the nonoxidizing gas is hydrogen characterized by a dew point not higher than minus 60 F.

References Cited UNITED STATES PATENTS 1,916,474 7/1933 French et al. 14820.3 2,303,132 11/1942 Murray et al. 148134 2,433,296 12/ 1947 Schaefer 22842 2,469,975 5/1949 McCloy 29422 CHARLES N. LOVELL, Primary Examiner. 

